Tight curves are where six-axle units can do a beating on the tracks. Weight often isn't the issue; per-axle weight is often the same, or less than, a four-axle unit. Example: SD40-2 at 390,000 LBs is 65,000 LBs per axle; GP40-2 at 270,000 LBs is 67,500 LBs per axle.
Now I'm not saying you'd want to run an SD40-2 or M636 on ancient 85 LB rail, but on 112 LB rail I'm sure it would be fine as long as the curves aren't too tight. I remember seeing Conrail's big six-axle GEs on Guilford's mainline in Maine back when that was still 112 LB stick rail (and poorly maintained at that).
Someone on the GuilfordRailSightings group mentioned that the "new" units coming to NBSR are GP38-3s from UP. They didn't cite a source for that information, though.
Pan Am Railways — Boston & Maine/Maine Central — Delaware & Hudson
Central Maine & Quebec/Montreal, Maine & Atlantic/Bangor & Aroostook
Providence & Worcester — New England — GE Locomotives